The design of highly slenderness rotors can be dominated by problems of aeroelasticity. These problems consist of two types depending on the source of excitation.
For small rotor frequencies in comparison with the inverse of the residence time (the time it takes for a fluid particle to pass through the rotor), the rotor may become aerodynamically unstable; under these circumstances, the amplitude of the vibration of the rotor is limited by friction. This phenomenon is known as limited amplitude flutter.
The second type of problems is known as forced response. In this case, the rotor is excited due to the wash of the preceding row and the amplitude of the vibration around the resonances is limited by aerodynamic damping and friction.
In both cases, increasing the aerodynamic damping has a beneficial effect on the life of the component.
There are several concepts for increasing the aerodynamic damping of rotors. Reference is herein made only to those that are related to the modification of the upper cover of the rotor or shroud. The main alternatives are the use of Z-shaped upper covers of the blade or rotor, commonly referred to as shrouds, making contact with the neighboring rotors and the use of the pairs of welded rotors (pairs of blades welded by the shroud).
Rotors with a Z-shaped upper cover for the blades, or shroud, and in contact with the adjacent rotors increase their stability essentially due to the increase of the natural frequencies provided by the support on the neighboring rotors. The main drawback is that they are difficult to precisely calculate and evaluate the influence of the wear of the contact over time which further introduces an additional dissipation. The pairs of welded rotors barely modify the natural frequencies of individual rotors, but not its mode shapes, which are very different. Its operating principle is that the welding prevents the relative movement between the blades of the pair for the most unstable modes, reducing virtually to zero the non-stationary pressure disturbances at the midway point of the passages between blades, significantly reducing the source of instability.
Both concepts have the same common point: they generate a single fundamental section that is repeated until the complete ring of the rotor is formed.
The Z-shaped interlock or shroud concept is widely used in the United States and Europe by all turbine manufacturers. The welded pair concept is only used by one turbine manufacturer, probably because its physical basis was not well understood until a short time ago.